1. Field of the Invention
The invention relates to polypropylene compositions having both improved impact strength and enhanced optical clarity.
2. Description of Related Art
Propylene polymers, e.g., crystalline polypropylene or crystalline copolymers of propylene with up to about 20 wt % of a C.sub.2 or C.sub.4 -C.sub.8 alpha-monoolefin such as ethylene, tend to be brittle (brittle point varies from 0.degree. C. to 20.degree. C.) and of poor impact strength at low temperatures. Polypropylene generally exhibits a notched Izod impact strength of less than about 1 ft.-lb/in. at room temperature.
It is generally known in the prior art that the brittle point of polypropylene can be lowered by blending the polymer with minor amounts of an elastomeric polymer such as natural rubber or GRS rubber, as disclosed in UK Patent 856,793.
UK Patent 950551 discloses that the impact properties of isotactic polypropylene are improved by forming blends of the polymer with up to about 28.5 wt % of polyisobutylene and up to about 5 wt % of polyethylene. U.S. Pat. No. 3,562,790 discloses blends of improved impact strength and clarity based on a ternary mixture of polypropylene, from about 2 to 15 wt % of polyethylene (or a copolymer of ethylene with a different alpha-olefin) and about 2 to 20 wt % of an ethylene/propylene/non-conjugated diene elastomer. U.S. Pat. No. 4,500,681 discloses ternary blends containing 20 to 65 wt % isotactic polypropylene, 20 to 60 wt % of an isobutytene polymer such as polyisobutylene, butyl rubber or halogenated butyl rubber and from about 7 to 40 wt % of a copolymer of ethylene and an unsaturated ester of a carboxylic acid such as vinyl acetate.
Similarly, EPA 214945 discloses polypropylene film materials having improved impact strength based on blends of polypropylene with minor amounts of one or more of low density polyethylene, polybutylene, polyisobutylene and/or copolymers of ethylene with methyl acrylate.
While many of these compositions and others give rise to some improvement in propylene polymer impact properties and result in a lowering of the brittle temperature, other properties may suffer, such as reduction in optical clarity with respect to blends containing polyisobutylene or polyolefins such as polyethylene, and a reduction in physical properties such as stiffness and processability with respect to other blends. In addition, the inclusion of relatively large amounts of modifying polymer(s) in blends with polypropylene tends to detract from the normally good tensile strength and modulus properties of pure polypropylene.
U.S. Pat. Nos. 5,013,793, 5,051,477 and 5,051,487 disclose dynamically vulcanizable polymer compositions based on a blend of an elastomeric isobutylene/para-methylstyrene interpolymer, preferably a halogenated interpolymer, one or more olefin polymers and a vulcanizing system for the isobutylene/para-methylstyrene interpolymer. The compositions are cured under high shear mixing conditions to provide a dispersion of fine particles of the cured elastomer dispersed in the olefin polymer matrix.